The present invention relates to measuring instruments and particularly to such instruments wherein proper connection with the device or structure under test can be verified.
An advantageous procedure for checking the integrity of metal structures involves low resistance measurements. For example, resistance measurements in the range of 1 milliohm are made on completed aircraft structures to verify proper assembly. This procedure is particularly important for checking the attachment of grounding studs and the like which may actually form a part of the aircraft's electrical system.
The measuring of low resistance values, however advantageous, is often accompanied by inaccuracies of large comparative magnitude and results are often non-repeatable. A measurement made at one stage in production may vary as much as 50% from a measurement made at another stage of production due to differing instruments, differing test leads, and differing contact resistances with the metal structure. A preferred resistance measuring device comprises a modified Kelvin bridge four terminal ohmmeter which essentially compares the unknown resistance with a known standard in the instrument and thereby lessens the effect of test leads and the like on the measuring accuracy. However, differing contact resistance encountered when measurements are undertaken at different times reduces accuracy. Probes and test leads are typically not checked frequently enough and non-conducting substances, wear and oxidation causes eventual increase in contact resistance. Flexing of the test leads results in increases in wire resistance due to individual strand breakage.
Moreover, intermittent contact between probes and the measured structure will sometimes form the basis for an apparently valid resistance reading since some meters tend to average open and closed circuit conditions. Furthermore, some four terminal ohmmeter designs display resistance values even when one or more of four connections from the meter to the tested structure are open circuit or when some of the test leads are connected to one another but not to a structure that is to be measured.
Prior attempts to provide more accurate readings have included frequent test of the ohm-meter instruments with a single or standard set of test leads. However, the same ohmmeter may then be employed with differing test leads and it is not always practical to measure the resistance of individual test leads in the field. Some custom design probes have been developed for making a better connection with specific measuring point geometries. Unfortunately, custom probes for every situation are not practical and contact resistance and lead resistance can still vary over time.
In some cases resistance standards have been included with ohmmeters in the field. The ohmmeter and test leads would be verified for operation by measuring the field resistance standard. This procedure is cumbersome and moreover contact resistances from the probes to an actual tested structure are often higher than the contact resistance to the field resistance standard.